Process for improving the drawability of a metal sheet or sheet blank

ABSTRACT

The process is characterized by the fact that, before drawing, a salt of the alkaline metal group formed by Li, Na, and K, and of an anion which is chemically inert with regard to the essential metal of the sheet chose from among the phosphates is deposited on the drawing tools. To deposit the salt, preferably, a solution of said salt is applied to the tools or to the sheet metal.

FIELD OF THE INVENTION

The present invention relates to a process for improving the drawabilityof metal, especially steel, sheets or sheet blanks, and to the use ofthis process either during the drawing or beforehand by treating thesheet intended to be drawn. It also relates to the sheet which is thustreated.

BACKGROUND OF THE INVENTION

The well-known principle of drawing consists in deforming plastically asheet blank held at its periphery between a cavity and a blank-clamp bymeans of a punch, the cavity and the punch having a configurationdetermined as a function of the desired shape of the drawn product.

The punch, the cavity and the blank-clamp, which are the members broughtinto contact with the blank during the drawing operation, will becalled, as a group, drawing tools in what follows.

In processes for shaping by plastic deformation, such as drawing, thesheet-tool friction coefficient plays an important part. It can limitthe ability of the sheet to deform and can result in ruptures of theblank if it is too high.

The lubrication, the state of the surface and the nature of the toolsare parameters which can be modified to reduce the friction.

During drawing, the surface of these tools which is in contact with theblank is lubricated, generally with a liquid lubricant or a drawing oil,to facilitate the deformation of the blank, to reduce the frictioncoefficient between blank and tools and thus to limit the risks ofseizure and of rupture of the blank.

However, this lubrication is not always sufficient to prevent theseizure and the rupture of the sheet blank, especially when the drawingpressure is high.

To solve these problems it has already been proposed to carry out atreatment of the surface of the sheet to be drawn. In particular, thereare known processes for chemical conversion of the surface of the sheet,such as phosphating, which make it possible to lower the sheet-toolfriction coefficient and thus promote the shaping of the blank. However,such a treatment is costly and cannot be applied to all the sheetsintended to be drawn.

A drawing process is also known from document U.S. Pat. No. 3,390,562,according to which a lubricating composition consisting of an oil and ofsalts is employed. However, this composition cannot be homogeneous. As aresult of this, the lubricating capacity of such a product is notuniform at all points of the sheet, because the salt is not distributeduniformly at its surface, and this interferes with the drawingoperation.

A disadvantage of these processes also lies in the fact that they resultin a modification of the chemical characteristics of a layer of metal atthe surface of the sheet. Although it may be tiny, this layer, whichremains at the surface after the component has been shaped, can be foundto interfere with the use of the component obtained. In particular,incompatibility may exist between the surface characteristics obtainedand the desired chemical treatments after shaping. Other disadvantagesrelating to the use of the components after shaping may also beprohibitive. For example, phosphating of the sheets constitutes aconsiderable impediment when the components which have been subjected toit are welded.

Another disadvantage of these processes lies in the weight and the costof the industrial plant needed for these surface treatments. A plant forphosphating sheets is costly, both owing to the size of the hardware ofwhich it consists and to its high energy requirements. In addition, thechemical conversion processes involve a minimum period of treatment andtherefore, in the case of a treatment in a continuous production line, aconsiderable length and bulkiness of the treatment plant.

Other attempts have been made, furthermore, to improve the lubricatingcharacteristics of drawing oils. However, the risk of seizure remains,especially in regions subject to high pressures, such as the tool edges,where the lubrication with such oils may be found insufficient.

SUMMARY OF THE INVENTION

The objective of the present invention is to solve these variousproblems and to do away with the abovementioned disadvantages.

With these objectives in mind, the subject of the invention is a processfor improving the drawability of metal, especially bare or coated steel,sheets or sheet blanks, characterised in that, before cold drawing,there is deposited on the sheet, or only on the drawing tool, a salt ofan alkali metal chosen from the group consisting of Li, Na and K and ofan anion which is chemically inert towards the metal of which the sheetconsists, chosen from phosphates, the deposition of the said salt beingcarried out by application to the said tools or the said sheet of asolution of the said salt, an oil being excluded.

According to a first application, the process is used during the drawingof metal sheet blanks by means of drawing tools comprising a cavity, ablank-clamp and a punch, and the deposition is carried out by applying asolution of said salt to the tools.

According to an alternative form of the invention, the application ofthe said solution is made to tools degreased beforehand. Drying of thetools may then be carried out. The tools may then be either oiled with adrawing lubricant, before the drawing of the blank is undertaken, or thepreoiled sheet blank may be drawn directly.

According to another alternative form, the application of the salt or ofthe solution is carried out on preoiled tools.

The application of the solution may be preferably carried out bydepositing drops of the said solution, which are distributed over thetools or localised in certain regions of the latter which areparticularly subject to seizing, such as the areas facing the cavity andthe blank-clamp or the most angular regions of the tools.

By virtue of the invention, the risks of seizure and of rupture of theblank can be considerably reduced or even eliminated. Moreover, thepressure of the blank-clamp can be raised without increasing this risk,and this improves the confinement of the edges of the blank between thecavity and the blank-clamp and prevents the formation of folds orundulations at these edges. Furthermore, in the case where a seizurewere nevertheless to occur during the drawing operation, the applicationof the said salt prevents the adhesion of scuffings to the tools, thatis to say of traces of the constituent metal of the sheet which is drawnand torn away therefrom during the eventual seizure. As a result, thecomponent drawn during the following operation does not run the risk ofbeing damaged by scuffings adhering to the tools.

An explanation which the inventors put forward for the observedimprovement in drawing is that the salt deposited on the tool forms asaponifying agent which, by reacting with the drawing oil, forms a soap,this reaction taking place especially by virtue of the pressure andtemperature conditions created at the surfaces of tool-blank contactduring the drawing.

According to another application of the process, the salt deposition maybe carried out on the sheet before drawing, or even during itsmanufacture.

Another subject of the invention is therefore also a metal sheetcharacterised in that it comprises on at least one face a film of asaponifying product causing the formation of a soap by reaction with anoil, this oil being of the type commonly employed during the drawingoperations. It will be noted that here and in the continuation of thisspecification the term "drawing" will be employed to denote generallyany processes for shaping sheet metal by plastic deformation.

The said product is preferably a salt of an alkali metal chosen from thegroup consisting of Na, K and Li, deposited on the surface of the sheetin a proportion of more than 10 milligrams per m² of area, and less than100 mg/m².

Also preferably, the said salt is a salt of an anion which is chemicallyinert towards the metal of which the sheet consists, chosen fromphosphates.

Another subject of the invention is a process for producing the abovesheet, this process being characterised in that the alkali metal salt isdeposited by applying to the surface of the sheet an aqueous solution ofthe said salt and the sheet thus treated is then dried to evaporate thesolvent.

This solution preferably contains from 10 to 100 g of the said salt perliter of water.

Also preferably, the solution contains potassium and phosphate ions andhydronium ions, and is neutralised.

The application of the solution to the sheet may be carried outespecially by spraying, immersion or coating, the sheet being driedimmediately after application of the solution.

The present invention, taken in one of its various embodiments set outabove, makes it possible to reduce considerably the value of thesheet-tool friction coefficient during the drawing, this being sowhether the sheets are bare or coated, for example galvanised, beforethe treatment.

When compared with the sheets referred to above, treated by chemicalconversion of their surfaces, the sheets according to the invention arefound to be very economical. Their process of manufacture allows anappreciable saving in space to be obtained with regard to the plantneeded for its implementation.

Other characteristics and advantages will appear in the descriptionwhich will be given by way of example, on the one hand of testsimplementing a process in accordance with the invention in the casewhere the said salt is deposited onto the drawing tools and, on theother hand, of comparative tests of sheets treated according to theinvention and then subjected to drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to the attached drawings, in which:

FIG. 1 is a graph showing two curves representing the frictioncoefficient as a function of pressure, these curves having beenestablished under the same operating conditions for treated anduntreated sheets respectively;

FIG. 2 is a graph showing, for different tests, the variation inblank-clamp pressure P causing the rupture of the blank, as a functionof the quantity Q of salt deposited on the sheet, expressed in mg ofpotassium per m².

The tests carried out by depositing the salt on the drawing tools willbe described first of all.

These tests were carried out with a conventional tool for drawing blanksof bare sheet and of sheet electrogalvanised on one face.

The deposition of salt on the tools was carried out by applying asolution of K₃ PO₄.2H₂ O diluted in water to concentrations of 5 to 20grams per liter.

A number of tests were carried out, consisting in drawing a number ofblanks with the same tool under different operating conditions, theratio of the diameter of the blank to the diameter of the punch being2.1 in all cases.

In each test the blank-clamp force needed to obtain rupture of the blankwas investigated.

The following tests were carried out in the case of each sheet grade,bare or coated on one face:

drawing according to the traditional process, without application of theK₃ PO₄ solution,

drawing according to the invention, with application of the K₃ PO₄solution to the punch, the cavity and the blank-clamp, which werecleaned and degreased beforehand, and by two methods, one consisting indrawing the blank without preliminary drying of the tool after theapplication and therefore allowing an excess of water to remain on thetools, and the other consisting in drawing the blank after drying thetool,

drawing according to the invention, with application of the solution topreoiled tools.

The results of these tests are shown in the table below, where theblank-clamp force needed to obtain rupture of the blank has been shownfor each of the abovementioned tests:

    ______________________________________                                                         K.sub.3 PO.sub.4.2H.sub.2 O                                                   on clean and                                                                  degreased tool                                                                with           K.sub.3 PO.sub.4.2H.sub.2 O                                                            Gain                                        Traditional                                                                             excess  after  on oiled in                                   Grades conditions                                                                              water   drying tool     %                                    ______________________________________                                        Bare   180 kN    240 kN  240 kN 240 kN   33                                   sheet                                                                         Sheet  120 kN    180 kN  180 kN 180 kN   50                                   coated                                                                        on 1 face                                                                     Cavity-                                                                       side                                                                          coating                                                                       ______________________________________                                    

Other tests were also carried out to determine the quantity of salt tobe deposited on the tools and needed to obtain a marked improvement indrawability when compared with traditional processes.

It can be seen that, by virtue of the process according to theinvention, the blank-clamp force before blank rupture is increased by33% in the case of the drawing of a bare sheet blank, and by 50% in thecase of coated sheet.

The process according to the invention makes it possible to increase theblank-clamp force considerably before rupture, and therefore the controlwidth of the drawing press. Thus, by virtue of the process according tothe invention, the blank-clamp force can be increased without increasingthe risk of seizure and rupture and it is therefore possible, forexample, to produce in a single pass drawn components which are deeperthan in the drawing processes according to the prior art, or to reducethe formation of folds or undulations in these components.

The results of these tests are listed in the table below, which shows,for each sheet grade and in the two cases of drawing according to theinvention, with application of salt to tools degreased beforehand, theconcentration of potassium in milligrams per m² of the tool area. Inthese tests a constant quantity of solution was deposited on the tools,the potassium concentration of this solution being varied.

    ______________________________________                                        Potassium concentration in mg/m.sup.2                                                      on the tool                                                                               After drying/                                                                 evaporation                                                         With excess                                                                             of surplus                                           Grades         water     water                                                ______________________________________                                        Bare sheet     350       200                                                  Coated sheet   150       100                                                  ______________________________________                                    

It can thus be concluded that in the case of drying after application ofthe solution to the tools, and therefore evaporation of the excesswater, the quantity of K₃ PO₄.H₂ O which is needed is markedly decreasedwhen compared with the case where the drawing is carried out without thetools being dried beforehand.

Furthermore, during these tests the inventors found that the use of asolution of relatively low potassium concentration and therefore, for anequal quantity of deposited salt, a larger quantity of solution andtherefore of water which is employed, delays the action of the salt andproduces a suction-cup effect between the cavity and blank-clamp whichinterferes during the drawing.

It is therefore preferable to perform an at least partial drying of thetools after the solution has been applied, both to reduce the quantityof salt which is needed and to improve the drawing, this drying beingpreferably carried out using pulsating hot air.

The inventors also found that the improvement in drawability due to theuse of the process according to the invention is practically immediateduring the drawing of coated sheet, whereas with bare sheet blanks thisimprovement appears only after drawing a number of blanks, carried outwith a reduced blank-clamp pressure.

Still further tests have made it possible to determine the durability ofthe effect of the salt application to the tool. To do this, apreliminary application of the solution was carried out and then anumber of blanks were drawn in succession without renewing theapplication, the drawing being then carried out with a blank-clamp forcecorresponding to approximately a half of the possible gain as determinedby the first series of tests (that is approximately 210 kN in the caseof the bare sheet). From 5 to 6 bare sheet blanks and from 10 to 12coated sheet blanks could thus be drawn without rupture.

As a result of these tests it has been found preferable, in the courseof a campaign of drawing according to the process of the invention, toemploy a solution of high concentration in order to start up morerapidly the process resulting in the improvement of drawability and,especially in the case of bare sheet drawing, to reduce the blank-clamppressure in relation to the pressure sought after during this campaign,on the first two or three blanks drawn after the initial application ofthe solution.

Of course, in the case of the application of the solution to toolsdegreased beforehand, this application does not eliminate the need forlubrication, it being possible for the latter to be carried out eitherby lubricating the tool after the solution has been applied or by usingpreoiled sheet blanks.

Tests carried out by applying salt to the sheet, followed by drawingblanks of the sheet thus treated will now be described.

The process according to the invention was tried out on bare sheets andon coated (galvanised) sheets. The inventors observed a more appreciableimprovement in the drawing characteristics in the case of coated sheets.

The experiments conducted by the inventors in the laboratory and onindustrial plant showed that untreated sheets stressed in drawing breakat a low blank-clamp pressure (40 bars) whereas these same sheetstreated according to the invention allow blank-clamp pressures of 140bars to be attained before rupture.

This results not only in the possibility of carrying out drawing at highpressure, but also in a considerable increase in the scope for drawing,that is to say the blank-clamp pressure range, which is limited at lowpressures by the appearance of folds in the drawn shape and at highpressures by tearing of the blank.

The table below illustrates the results obtained during drawing tests onsamples obtained by five samplings. One half of the samples (A) fromsamplings 1, 2 and 3 having been treated in the laboratory, and thesamples (B) from the samplings 4 and 5 treated on industrial productionlines.

Each sample is a steel sheet blank on which sodium phosphate isdeposited by coating the surface of the sheet with an aqueous solutioncontaining 60 g of sodium phosphate per liter, so that the quantity ofsodium phosphate is 20 mg/m², and then drying the sheet and, finally,coating it with an ordinary protective oil.

    ______________________________________                                               Blank-clamp pressure at which                                                 breakage occurs (bars)                                                 Reference                                                                              Without treatment                                                                            After treatment                                       ______________________________________                                        1        40                         >100                                      1        40                         >140                                      1        40             A-          >140                                      2        40                         120                                       3        40                         120                                       4        60                         160                                       4        60                         160                                       4        60                         180                                       5        40             B-          140                                       5        40                         140                                       5        40                         160                                       ______________________________________                                    

Other tests have made it possible to obtain the curves in the graph ofFIG. 1, showing the variation in the friction coefficient as a functionof pressure. These curves were obtained by measurement on a conventionaltribometer with parallel surfaces, with sheets electro-galvanised onboth faces, the coating of which has a thickness of 10 μm.

It is easily concluded that, in the case of the treated sheet, thefriction coefficient is overall markedly lower than in the case of theuntreated sheet.

The graph in FIG. 2 shows the result of other tests carried out whilevarying the quantity Q of salt, potassium phosphate of formula K₃ PO₄ inthis case, deposited on the sheet (on one face only), expressed in mg ofpotassium per m².

It is found that, in the case of quantities of less than approximately 5mg/m², the blank-clamp pressure values P resulting in rupture of theblank are scattered, in many cases with rupture in the case of pressuresclose to 40 bars, whereas in the case of quantities of more than 5 mg/m²no rupture is observed up to 120 bars.

It should be remembered that, in the case where the salt deposition iscarried out on the sheet or on the blank, this treatment obviously doesnot do away with the necessity of oiling the sheets before drawing,since the improvement in drawability obtained by virtue of the inventionresults from the combination of the products deposited in accordancewith the invention and of the oil commonly employed during the drawingoperation.

I claim:
 1. A process for improving the cold drawability of metalconsisting essentially of, before cold drawing, depositing on a drawingtool a salt consisting essentially of an alkali metal chosen from thegroup consisting of Li, Na, and K and an anion which is chemically inerttowards the metal of which the sheet consists, chosen from phosphates,wherein said deposition step of said salt is carried out by application,to said tool, of a solution of said salt, wherein said solution does notcomprise oil.
 2. A process according to claim 1, wherein saidapplication of the solution is to certain regions of said tool.
 3. Aprocess according to one of claims 1 or 2, wherein said deposition stepis carried out on a tool which has been previously degreased.
 4. Aprocess according to claim 3, wherein said tool is at least partiallydried after said solution has been applied.
 5. A process according toclaim 3, wherein said tool is oiled after application of the solution.6. A process according to claim 3, further comprising drawing of apreoiled sheet blank.
 7. A process according to one of claims 1 or 2,wherein said solution is applied to an oiled tool.
 8. A processaccording to claim 1, wherein said application step is carried out bydepositing drops of said solution distributed over said tool.
 9. Aprocess according to claim 1, further comprising drawing more than oneblank after application of the solution, prior to reapplying saidsolution.
 10. A process as claimed in claim 1 wherein said metal isselected from bare or coated steel, sheets, and sheet blanks.
 11. Aprocess as claimed in claim 1, wherein said solution consists of waterand 5-20 g/l of K₃ PO₄.2H₂ O.
 12. A method to improve the colddrawability of metallic sheets or sheet blanks, consisting essentiallyof applying to the surface of said sheets or sheet blanks, and aqueoussolution consisting essentially of an alkali metal salt chosen from thegroup consisting of Li, Na and K and of an anion which is chemicallyinert towards the metal of which the sheet is formed, wherein said anionis a phosphate, wherein said aqueous solution does not contain oil, andwherein after said solution has been applied said solution is dried. 13.The method of claim 12, wherein said solution further compriseshydronium ions.
 14. A method according to claim 12, wherein said aqueoussolution comprises potassium phosphate, and 5-20 g/l of K₃ PO₄.2H₂ O.15. A method according to claim 12, wherein said solution comprisespotassium phosphate K₃ PO₄, and wherein after said solution has beenapplied, said solution is dried, wherein after drying the amount of saltuniformly deposited is greater than 5 mg/m² of potassium.
 16. A methodas claimed in claim 12 wherein said metallic sheets or sheet blanks aremade of bare or coated steel.
 17. A method as claimed in claim 12,wherein said aqueous solution consists of water and 5-20 g/l of K₃PO₄.2H₂ O.